Boxing Nullable Types (C# Programming Guide)

Objects based on nullable types are only boxed if the object is non-null. If HasValue is false, the object reference is assigned to null instead of boxing. For example:

bool? b = null;
object o = b;
// Now o is null.

If the object is non-null -- if HasValue is true -- then boxing occurs, but only the underlying type that the nullable object is based on is boxed. Boxing a non-null nullable value type boxes the value type itself, not the System.Nullable<T> that wraps the value type. For example:

bool? b = false;
int? i = 44;
object bBoxed = b; // bBoxed contains a boxed bool.
object iBoxed = i; // iBoxed contains a boxed int.

The two boxed objects are identical to those created by boxing non-nullable types. And, just like non-nullable boxed types, they can be unboxed into nullable types, as in the following example:

bool? b2 = (bool?)bBoxed;
int? i2 = (int?)iBoxed;

Remarks

The behavior of nullable types when boxed provides two advantages:

1. Nullable objects and their boxed counterpart can be tested for null:

     bool? b = null;
     object boxedB = b;
     if (b == null)
     {
       // True.
     }
     if (boxedB == null)
     {
       // Also true.
     }
   

2. Boxed nullable types fully support the functionality of the underlying type:

     double? d = 44.4;
     object iBoxed = d;
     // Access IConvertible interface implemented by double.
     IConvertible ic = (IConvertible)iBoxed;
     int i = ic.ToInt32(null);
     string str = ic.ToString();
   

For more examples of nullable types and boxing behavior, see Nullable Sample.

See Also

Tasks

How to: Identify a Nullable Type (C# Programming Guide)

Concepts

C# Programming Guide

Reference

Nullable Types (C# Programming Guide)